Here are a few resources you can use for a variety of games. This list is not comprehensive.

Gaming Systems by Genre

Fantasy

• Ars Magica 4th Edition free
• BareBones Fantasy by DWD Studios (not to be confused with the Bare Bones Multiverse)
• Basic Fantasy RPG
• Castles & Crusades
• Dungeons & Dragons
• Dungeons & Dragons 3.5E System Reference Document
• Dungeons & Dragons 5th-edition , free download of basic rules for players and Dungeon Masters
• Dungeon World
• Fantasy AGE
• Pathfinder
• Pathfinder Reference Document

Superhero

• Mutants and Masterminds
• Marvel Superheros
• Champions
• City of Mist
• Masks
• Four Color System free

Modern / Spy / Military

• Covert Ops by DWD Studios
• d20 Modern - by Wizards of the Coast
• Feng Shui- based on Hong Kong/martial arts movies
• GUMSHOE System - by Pelgrane Press
• James Bond 007 - by Victory Games [out of print] - based on the Ian Fleming character
• Mercenaries, Spies and Private Eyes - Designed by Michael Stackpole.
• Over the Edge - by Atlas Games
• Spycraft - by Crafty Games - espionage/adventure
• White Lies by DWD Studios (espionage with "white box" D&D-type rules)
• Modus Operandi - Espionage RPG webring - for all things spy...

Sci-Fi

• Danger Patrol
• Star Frontiers (free remastered edition)
• Traveller (editions edited by Marc Miller)
• Traveller (Mongoose Publishing edition, soon to go to 2nd edition)
• Star Wars: The Roleplaying Game(WEG D6) (Community created & open license rulebooks)

Horror

• Call of Cthulhu
• World of Darkness

Generic or Multi-genre

• Fiasco
• GURPS
• Hero System
• Shadowrun
• Open D6 Books SRD D6 game conversions
• FATE

Rules-light

• Risus: The Anything RPG free
• Lasers and Feelings free
• Mini Six: Bare Bones Edition free
• Hero Kids

Libraries

• http://www.opengameart.org - Graphics, Sound Effects, etc.
• http://rpgmapshare.com - maps
• http://www.game-icons.net/ - online repository providing heaps of cool game related graphics(Creative Commons)

Graphics & Mapping tools

See also: Useful links for mapping

• http://www.blender.org - open source 3d graphics software
• http://www.gimp.org - open source 2d graphics software
• http://pyromancers.com - free online map maker
• http://rpgvirtualtabletop.wikidot.com/tokens - tokens (free & paid)
• Hero Poirtrait Creator
• http://www.rptools.net/index.php?page=tokentool - Token Creator
• http://sourceforge.net/projects/pymapper/ - Map Maker
• http://rpgvirtualtabletop.wikidot.com/mapping-software - List of various mapping tools
• http://www.gozzys.com - Random Map Generators for Dungeons, Caves, and Wilderness.
• http://donjon.bin.sh - Random Generators including Maps of various varieties.

Sounds

• http://audacity.sourceforge.net - open source sound recorder/editor
• http://www.freesound.org - sounds
• http://www.pdsounds.org - public domain sounds - sound library
• http://sounddogs.com/ - sound effects
• http://tabletopaudio.com/ - Original, 10 minute ambiences and music-loops for your tabletop role-playing games. These loops are now integrated into the Roll20 Jukebox.

There are also SoundPads which are mixing-boards of ambience sounds and music, which can be Broadcast to your players (independently of Roll20).

System Specific Tools

D&D 4E

• D&D 4E character sheet service
• Treasure Generator
• Treasure Index
• Combat Manager
• "Drama" / "Bonus" Cards
• D&D 4E Info Tracker
• http://donjon.bin.sh/4e/ - random generators

D&D 5E

• http://donjon.bin.sh/5e/ - random generators

Pathfinder

• http://donjon.bin.sh/pathfinder/ - random generators
• http://pathfinder-treasure-generator.com/ - treasure generator

Traveller 5th-edition

• http://heldenhaufen.de/T5 - SectorMaker, SystemMaker, BeastMaker, GunMaker
• http://eaglestone.pocketempires.com/ - several utilities including Task Roller and ship conversions to T5.

Star Wars D6

• Quick NPC Generator downloadable exe
• Ship Price Calculator online
• Hyperspace Traveltime Calculator online, system agnostic

Books

• XDM X-Treme Dungeon Mastery, by Tracy Hickman
• The Lazy Dungeon Master, by Sly Flourish
• Gamemastering By Brian Jamison 330 pages, free
• Grain Into Gold, A Fantasy World Economy A Simple and Sane Supplement for use with your Fantasy Role-Playing Game

Game Master articles

• The Alexandrian, Game Mastery 101
• Sly Flourish, General RPG Advice & DM Deep Dive
• The Angry GM, RPG Advice with Attitude
• Campaign Mastery,Expert advice on creating and running exceptional campaigns.
• The Complete Hippo Massive collection of articles in Gming by user famoushippopotamus
• Economicon Thoughts about D&D fantasy economy and it's implications

Youtube Channels

• How to be a Great Game Master channels
• Take20 GM Tips channels
• Matthew Colville Running the Game 58 video playlist
• Web DM GM tips focused on D&D5e

See Also

• Game Resources in the Off-Topic forum archive